a) Field of the Invention
The invention is directed to an arrangement for stabilizing the radiation emission of a plasma, particularly for generating extreme ultraviolet radiation (EUV radiation), in which a bundled energy beam is directed to a target, wherein the target is formed as a target jet and has a flow direction oriented substantially orthogonal to the radiating direction of the energy beam.
b) Description of the Related Art
A laser-produced plasma (LPP) is usually used to generate EUV radiation. For this purpose, a target, as it is called, is irradiated by a laser and in this way is heated to the extent that characteristic and temperature radiation with a significant proportion in the extreme ultraviolet (EUV) spectral region is emitted. In practice, the target is struck by the laser beam differently over the course of time resulting in fluctuations in the intensity of the EUV radiation over time. However for many applications, especially as a radiation source for semiconductor lithography, the emitted radiation output in the EUV spectral region may only be subject to very slight fluctuations.
Some prior art solutions use a target in the form of a target beam which ensures a continuous material flow with the highest possible density and low divergence. A target beam of this type typically has a diameter between 0.01 mm and 0.1 mm. A laser beam directed to the target beam must be focused in order to generate the plasma. Vibrations or other variations in the two (relatively) independent generation systems typically lead to directional instabilities of the target beam and laser beam and, accordingly, to an absorption efficiency of the laser energy into the target material that varies over time and, therefore, to an irregular emission of the EUV radiation.
Devices and methods for EUV generation by means of laser irradiation of different targets are described in numerous patents and laid-open applications. Many of these targets, particularly the mass-limited targets, have in common that they have small dimensions in the sub-millimeter range in two dimensions, e.g., H. Hertz WO 97/40650/EP 0895706 (jet target), M. Schmidt WO 01/30122 A1 (droplet mist), or even in three dimensions, e.g., E. Noda EP 0186491 B1 (droplet). The constancy of the absorption efficiency of the laser radiation into the target over time is not monitored or ensured in any of the references cited above.